Amorphous alloys (or metallic glasses) have no discernable pattern existing in their atomic structure in contrast to ordinary crystalline metals and alloys. This unique atomic structure results in very high yield strengths and high hardnesses for amorphous alloys. These superior properties are generally attributed to the lack of the dislocations typically found in crystalline atomic structures. In addition, amorphous alloys generally have high elastic strain limits approaching, up to 2.0%, much higher than any other metallic alloys. For example, the yield strength of Ti-base amorphous alloys is about 2 GPa or more, values exceeding the current state of crystalline titanium alloys. Finally, amorphous alloys can be formed by a variety of methods among which quenching from the liquid state is the most common and widely used method.
However, amorphous alloys in bulk forms (alloys capable of being formed with a minimum dimension of at least 0.5 mm, which are also referred to as bulk-solidifying amorphous alloys or bulk amorphous alloys) have some shortcomings which result in reduced utilization of the high yield strength and high elastic strain limit properties of these materials in load bearing structural applications. First, the sensitivity of amorphous alloys to defects and their low resistance to crack propagation from defects are primary causes of premature failure. For example, the fatigue endurance limit of amorphous alloys can be quite low, and values as low as 10% of its ultimate strength have been reported. In the case of high stress-low cycle cases, amorphous alloys generally fail around 50% of their ultimate strength after several thousands cycles. This is generally attributed to the “micro-structureless” nature of the amorphous phase and the lack of any of the work-hardening mechanisms typically found in crystalline alloys.
Accordingly, the conventional work hardening and strengthening methods generally used for crystalline alloys have been deemed to be non-applicable to amorphous alloys. Therefore, some composite forms of amorphous alloys have been developed to remedy the shortcomings of toughness and fatigue resistance. Although these composites show improvement in the toughness and durability (or fatigue resistance) it can only be done at the expense of the high yield strengths and the high elastic strain limits of the pure amorphous alloy materials, and as such defeat the principal benefits of using these materials. Accordingly, there is a need for amorphous alloys with improved durability and fatigue resistance that are capable of maintaining relatively high yield strengths and high elastic limits in loading bearing structural applications.